4e-inspired THAC0/AC system

The THAC0/combat table and AC system is, with the Saving Throws and skill system, one of the chief bones of contention, as it is often considered awkward.

I've taken into consideration the 4e system (a single progression with semi-static bonuses for Fighters), and have tried to generate a similar system for OD&D (i.e., one that produces more or less the same hit probabilities and the same difference between fighters and other classes).

The system is not playtested, but should be sound.

Character base attack progressionAll characters, regardless of class, have a base attack bonus equal to 1/2 their level, rounded up

Bonus damageIn OD&D, high level characters do bonus damage when fighting enemies they could automatically hit, if it wasn't for the "1 always misses" rule. The same is easily obtained here, with a rule similar to power attack of newer editions:

If AB > AC : AB-AC damage bonus

ArmorObviously, AC is reversed to simplify the computations. AC grows from 11 up, and is computed as 20 - Old AC

It's easy to see that hit rate improves slightly with tiers, which is IMO a good thing, as it offsets the increased amount of HP.
At 36th level, a Fighter should have a base bonus of 24 (2/3 his level), if one used the OD&D progression. Here, he arrives at 23. A cleric or thief gets exactly the same bonus as in the standard table (basically, the cleric progression is extended to all characters), except their progression is at a a finer grain (+1 every 2 levels instead of +2 every 4).
A Magic User gets an improved hit probability (+17 instead of +15), but not by much.

Demihumans need to progress up to 36th level, and are similar to Fighters at basic/expert levels, and to Clerics at higher levels, as in standard OD&D. Halflings are more difficult to mirror, since they progress as Fighters at lower level and as Magic users at higher levels in standard OD&D, and are therefore changed to progress as Clerics (this should be also more balanced).

The main difference between this progression and the original OD&D one is in the nonlinearity that you have when low level characters attempt to hit very low AC, which is different. However, I don't think it is worth reproducing, since low-level characters would be killed in short order by anything that had AC -5 or better...